better error message for React EventPlugin order check

Ran into this while inadvertently requiring multiple React versions in a separate project (`require('react');` vs `require('React');`

src/event/EventPluginRegistry.js

@@ -176,7 +176,8 @@ var EventPluginRegistry = {
   injectEventPluginOrder: function(InjectedEventPluginOrder) {
     invariant(
       !EventPluginOrder,
-      'EventPluginRegistry: Cannot inject event plugin ordering more than once.'
+      'EventPluginRegistry: Cannot inject event plugin ordering more than ' +
+      'once. You are likely trying to load more than one copy of React.'
     );
     // Clone the ordering so it cannot be dynamically mutated.
     EventPluginOrder = Array.prototype.slice.call(InjectedEventPluginOrder);

src/event/__tests__/EventPluginRegistry-test.js

@@ -135,7 +135,8 @@ describe('EventPluginRegistry', function() {
       EventPluginRegistry.injectEventPluginOrder(pluginOrdering);
     }).toThrow(
       'Invariant Violation: EventPluginRegistry: Cannot inject event plugin ' +
-      'ordering more than once.'
+      'ordering more than once. You are likely trying to load more than one ' +
+      'copy of React.'
     );
   });